The present invention relates to a method for assembling a piston-and-cylinder unit and a force transmission element, for instance a pressure element, with the unit comprising a first piston (push rod piston) as well as a second piston (floating piston) connected downstream of the first piston in the actuating direction, as well as comprising each one central valve allocated to the first and second pistons, respectively, which separates the pressure chamber assigned to the push rod piston and to the floating piston from an unpressurized brake fluid supply reservoir and/or connects it to the brake fluid supply reservoir in dependence on the piston's movement. Such central valves are described for instance in German published patent application (DE-OS) 36 29 564. This application discloses a brake system with slip control essentially consisting of a pedal-operated, preferably auxiliary-energy-assisted braking pressure generator which the wheel brakes are connected to via pressure lines, of a hydraulic auxiliary-pressure supply system with a hydraulic pump, with a pressure-compensating and pressure fluid supply reservoir and with an auxillary-pressure control valve as well as wheel sensors and electronic circuits for determining the rotational behavior of the wheels and for generating electric braking pressure control signals by means of which electromagnetically operable pressure fluid inlet valves and outlet valves are controllable which are inserted into the pressure fluid lines for the purpose of slip control.
The subject matter of the cited published patent application is characterized by a switch inserted into the pressure line of the auxiliary energy source, said switch operating upon the attainment of a predetermined hydraulic pressure and thus emitting an electric signal to the signal processing and combining circuit which latter causes the pump's motor to switch on and off for a checking cycle and which thus can signalize the driver of the vehicle and motor-and-pump unit's readiness for operation via a visual or acoustic display.
When combining master cylinders and vacuum brake power boosters as described in the cited published patent application as well as, generally, when assembling a piston-and-cylinder unit and a force transmission element, with one valve each being arranged in serially arranged pistons of the unit, there will result the disadvantages described in the following and explained with reference to the central valves of the subject matter of the above-mentioned published patent application.
The closing travel of the central valves of the two pressure circuits and the lost travel associated therewith becomes too large by assembly. Said excessive lost travel results from the adding-up of many tolerances of the individual components of the tandem master cylinder. When multiplying this closing travel with the pedal transmission, shortcomings are encountered when the pedal is operated.
According to the prior art, the tandem master cylinder is assembled, except for the spacing washers and the pressure element. Then, the gauge is ascertained which serves as a distance between the surface of the flange of the master cylinder, on the one hand, and the bore depth of the pressure element, on the other hand, said flange serving to connect the vacuum brake booster. The number of spacing washers are determined in a further step. The spacing washers and the pressure element with retaining ring will be assembled subsequently.
In this way, in prior art it is admittedly achieved that the tandem master cylinder and the vacuum booster are assembled free from play. On the other hand, however, all the tolerances of the individual components fully add to the central valve closing travel in a very disadvantageous manner. The lost travel of the central valves becomes correspondingly large. As illustrated above, it disadvantageously multiples with the pedal transmission.